1. Field of the Invention
The present invention relates to a transmission circuit used for a communication apparatus such as a mobile phone, a wireless LAN device or the like, and more specifically to a compact transmission circuit for outputting a highly linear transmission signal regardless of the level of the output power and operating at a high efficiency, and a communication apparatus using the same.
2. Description of the Background Art
A communication apparatus such as a mobile phone, a wireless LAN device or the like is required to guarantee the linearity of the transmission signal while operating at a low power consumption whether the communication apparatus may operate at a high output power or a low output power. For such a communication apparatus, a compact transmission circuit for outputting a highly linear transmission signal and operating at a high efficiency regardless of the level of the output power is used. Conventional transmission circuits will be described below.
An exemplary conventional transmission circuit generates a transmission signal using a modulation system such as, for example, quadrature modulation (hereinafter, referred to as a “quadrature modulation circuit”). The quadrature modulation circuit is widely known and will not be described here. Another conventional transmission circuit for outputting a highly linear transmission signal at a higher efficiency than the quadrature modulation circuit is, for example, a transmission circuit 500 shown in FIG. 21. FIG. 21 is a block diagram showing a structure of the conventional transmission circuit 500. As shown in FIG. 21, the conventional transmission circuit 500 includes a signal generation section 501, an angular modulation section 502, an amplitude amplification section 503, an amplitude modulation section 504, and an output terminal 505.
In the conventional transmission circuit 500, the signal generation section 501 generates an amplitude signal and a phase signal. The amplitude signal is input to the amplitude amplification section 503. The amplitude amplification section 503 supplies a voltage controlled in accordance with the level of the input amplitude signal to the amplitude modulation section 504. The phase signal is input to the angular modulation section 502. The angular modulation section 502 performs angular modulation on the input phase signal, and outputs the resultant signal as an angle-modulated signal. The angle-modulated signal is input to the amplitude modulation section 504. The amplitude modulation section 504 performs amplitude modulation on the angle-modulated signal with the voltage supplied from the amplitude amplification section 503, and outputs the resultant signal as an angle-modulated and amplitude-modulated signal. This modulation signal is output from the output terminal 505 as a transmission signal. Such a transmission circuit 500 is referred to as a polar modulation circuit.
Still another conventional transmission circuit for outputting a highly linear transmission signal at a higher efficiency than the quadrature modulation circuit is, for example, a transmission circuit 600 which is referred to as a LINC (Linear Amplification using Nonlinear Components) circuit and shown in FIG. 22. FIG. 22 is a block diagram showing a structure of the conventional transmission circuit 600. As shown in FIG. 22, the conventional transmission circuit 600 includes a constant amplitude wave generation circuit 601, an amplification section 602, an amplification section 603, and a combining circuit 604.
The constant amplitude wave generation circuit 601 outputs two modulated signals having different phases and a constant amplitude (hereinafter, referred to as “constant amplitude signals”) based on an input signal. The two constant amplitude signals which are output from the constant amplitude wave generation circuit 601 are amplified by the amplification sections 602 and 603 and input to the combining circuit 604. The combining circuit 604 combines an output signal s1 from the amplification section 602 and an output signal s2 from the amplification section 603, and outputs the combined signal as a transmission signal s0.
The transmission signal s0, the output signal s1 from the amplification section 602, and the output signal s2 from the amplification section 603 are represented by expressions (1) through (4). In expressions (1) through (4), M(t) represents an amplitude component of the transmission signal s0, and θ(t) represents the phase component of the transmission signal s0. Mx represents the level of the output signal s1 from the amplification section 602 and the level of the output signal s2 from the amplification section 603. ψ(t) represents the phase shift of the output signal s1 and the output signal s2 with respect to the transmission signal s0.
                              s          ⁢                                          ⁢          0          ⁢                      (            t            )                          =                                            M              ⁡                              (                t                )                                      ⁢                          exp              ⁡                              [                                  jθ                  ⁡                                      (                    t                    )                                                  ]                                              =                                    s              ⁢                                                          ⁢              1              ⁢                              (                t                )                                      +                          s              ⁢                                                          ⁢              2              ⁢                              (                t                )                                                                        expression        ⁢                                  ⁢                  (          1          )                                                  s          ⁢                                          ⁢          1          ⁢                      (            t            )                          =                  Mx          ⁢                                          ⁢                      exp            ⁡                          [                              j                ⁢                                  {                                                            θ                      ⁡                                              (                        t                        )                                                              +                                          ϕ                      ⁡                                              (                        t                        )                                                                              }                                            ]                                                          expression        ⁢                                  ⁢                  (          2          )                                                  s          ⁢                                          ⁢          2          ⁢                      (            t            )                          =                  Mx          ⁢                                          ⁢                      exp            ⁡                          [                              j                ⁢                                  {                                                            θ                      ⁡                                              (                        t                        )                                                              -                                          ϕ                      ⁡                                              (                        t                        )                                                                              }                                            ]                                                          expression        ⁢                                  ⁢                  (          3          )                                                  ϕ          ⁡                      (            t            )                          =                              cos                          -              1                                ⁡                      [                                          M                ⁡                                  (                  t                  )                                                            2                ⁢                Mx                                      ]                                              expression        ⁢                                  ⁢                  (          4          )                    
FIG. 23 specifically illustrates an operation of the conventional transmission circuit 600. The conventional transmission circuit 600 reduces the phase shift of the output signal s1 and the output signal s2 with respect to the transmission signal s0, and thus outputs a high transmission signal s0 (see FIG. 23, (a)). The conventional transmission circuit 600 enlarges the phase shift of the output signal s1 and the output signal s2 with respect to the transmission signal s0, and thus outputs a low transmission signal s0 (see FIG. 23, (b)). Namely, the transmission circuit 600 can control the phase shift of the two constant amplitude wave signals which are output from the constant amplitude generation circuit 601 to control the level of the transmission signal s0.
However, the conventional transmission circuit 600 generates a transmission signal s0 by combining the output signals s1 and s2. Therefore, when the output signal s1 or s2 includes a phase error or an amplitude error, it is difficult to output a desired transmission signal s0.
A conventional LINC transmission circuit for correcting the phase error or the amplitude error included in the output signal s1 or s2 is disclosed (for example, see Japanese Laid-Open Patent Publication No. 5-37263; hereinafter, referred to as “patent document 1”). FIG. 24 is a block diagram showing a structure of a conventional transmission circuit 700 disclosed in patent document 1. As shown in FIG. 24, the conventional transmission circuit 700 includes a constant amplitude wave generation circuit 601, an amplification section 602, an amplification circuit 603, a combining circuit 604, a phase detector 701, a variable phase device 702, an amplitude difference detector 703, and a variable attenuator 704.
In the conventional transmission circuit 700, the phase detector 701 detects a phase error included in the output signal s1 from the amplification section 602. The variable phase device 702 corrects the phase of the constant amplitude signal generated by the constant amplitude wave generation circuit 601 based on the detected phase error. The amplitude difference detector 703 detects an amplitude error included in the output signal s2 from the amplification section 603. The variable attenuator 704 corrects the amplitude of the constant amplitude signal generated by the constant amplitude wave generation circuit 601 based on the detected amplitude difference. Thus, the conventional transmission circuit 700 can output a desired transmission signal s0.
The conventional transmission circuit 500 (FIG. 21) has a problem of not being able to output a transmission signal having a level lower than that of a predetermined output power (i.e., there is a lower limit on the output power of the transmission signal). FIG. 25 shows an example of the output characteristics of the conventional transmission circuit 500. In FIG. 25, the horizontal axis represents the amplitude of the signal which is output from the signal generation section 501. The vertical axis represents the output power of the transmission signal. As shown in FIG. 25, in the conventional transmission circuit 500, the amplitude modulation section 504 cannot easily operate linearly in an area where the output power is low (i.e., where the level of the amplitude signal is low). Therefore, a highly linear transmission signal cannot be output in such an area.
The conventional transmission circuit 600 (FIG. 22) has the problem that because a transmission signal s0 is generated by combining two output signals s1 and s2 having different phases, it is difficult to output a desired transmission signal s0 due to a phase error or an amplitude error included in the output signal s1 or s2 as described above. The conventional transmission circuit 600 also has a problem that because a transmission signal s0 is generated by combining two output signals s1 and s1 having different phases, the operation efficiency is not guaranteed to be high when the level of the output power is of a certain value.
The conventional transmission circuit 700 (FIG. 24) requires many elements (for example, the phase detector 701, the variable phase device 702, the amplitude difference detector 703, and the variable attenuator 704) in order to correct the phase error or the amplitude error included in the output signal s1 or s2. Therefore, the conventional transmission circuit 700 has a problem of being large in circuit scale. The conventional transmission circuit 700 also has a problem that because the outputs from the amplification sections 602 and 603 are each branched, a loss is generated and the power consumption of the conventional transmission circuit 700 is increased.